The move, aimed at bolstering the nation’s struggling coal industry, will trigger an immediate court fight and could result in months, if not years of litigation. Yet the policy reversal is unlikely to affect the nation’s overall shift from coal to natural gas and renewable power generation in the electricity sector.

“We are committed to righting the wrongs of the Obama administration by cleaning the regulatory slate,” Pruitt said in a statement. “Any replacement rule will be done carefully, properly, and with humility, by listening to all those affected by the rule.”

The Obama-era climate rule, dubbed the Clean Power Plan, aimed to cut carbon emissions from the nation’s electricity sector 32 percent by 2030, compared to 2005 levels. That measure, which the Supreme Court stayed after 28 attorneys general and the utility industry challenged it in court, would have required states to meet targets by reducing emissions at individual plants as well as by deploying renewable energy and taking other energy-efficiency steps.

As Oklahoma attorney general, Pruitt argued that EPA exceeded its statutory authority under the Clean Air Act in requiring utilities to pursue emissions reductions beyond their actual facilities. The notice of proposed rulemaking does not indicate whether EPA will replace the Clean Power Plan with a new rule, though the agency determined in 2009 that it was obligated to regulate carbon dioxide as a pollutant under the law because it endangered public health.

“We can now assess whether further regulatory action is warranted, and, if so, what is the most appropriate path forward, consistent with the Clean Air Act and principles of cooperative federalism,” Pruitt said. The previous rule, he added, “ignored states’ concerns and eroded long-standing and important partnerships that are a necessary part of achieving positive environmental outcomes.”

Although the 2014 climate rule never took effect, coal continues to decline as a source of the nation’s electricity. Coal accounted for about 30 percent last year, according to the U.S. Energy Information Administration, it accounted for about 30 percent of output last year, putting it below natural gas (nearly 34 percent) for the first time. Non-fossil-fuel generation (almost 36 percent) outranked fossil fuels for the first time since World War II.

A new analysis by the Rhodium Group estimates that CO2 emissions from the U.S. power sector in 2025 will be 27 percent to 35 percent below 2005 levels. But the same group projects that absent new federal or state action, the nation is on track to fall short of long-term climate targets it adopted under the 2015 Paris climate agreement. As part of that global accord, which President Trump has vowed to exit, the United States pledged to cut total carbon emissions between 26 percent and 28 percent by 2025.

John Larsen, the Rhodium Group’s director, said the rule’s repeal would not alter broader energy trends but could affect between 12 and 21 states not already on track to meet its targets.

“If a state wasn’t already acting to scale back emissions, the Clean Power Plan would nudge them to do that,” he said.

Many top Republicans on Capitol Hill hailed Pruitt’s announcement, including Senate Environment and Public Works Committee Chairman John Barrasso (Wyo.). “It would have hurt energy workers in Wyoming and harmed the state’s economy,” Barrasso said.

But several business and environmental groups decried the rollback, saying it would impede the deployment of clean energy and other measures, such as EPA’s Clean Energy Incentive Program, which focuses on improving energy efficiency in low-income communities.

“Undoing this plan would be a step backward for energy efficiency and the benefits it delivers for consumers and businesses alike,” Kateri Callahan, president of the Alliance to Save Energy, said in a statement.

Some states and corporations say they will maintain their efforts to reduce greenhouse gases, despite the policy changes underway in Washington. Mars, Inc. which has pledged to make its direct operations carbon-free by 2040, said it will continue to advance that goal.

At least four Democratic attorneys general — from California, Massachusetts, North Carolina and Oregon — have pledged to challenge EPA’s proposed rule in federal court, along with environmental groups such as Earthjustice.